A prosecutor said her office is considering pursuing the death penalty. Eighteenth Judicial District Attorney Carol Chambers said a decision will be made in consultation with victims’ families.

Holmes has been held in solitary confinement since the shooting.

Holmes began buying guns nearly two months before the shooting and recently bought 6,000 rounds of ammunition over the Internet, Aurora Police Chief Dan Oates said.

During the attack, Holmes allegedly set off gas canisters and used a semiautomatic rifle, a shotgun and a pistol to open fire, Oates said.

The semiautomatic assault rifle jammed during the attack, forcing the gunman to switch to another gun with less firepower, a federal law enforcement official told The Associated Press. That malfunction and weapons switch might have saved some lives.

Investigators have said they found a Batman mask inside the apartment.

As authorities rushed to piece together Holmes’ background, the owner of a gun range told The Associated Press that Holmes applied to join the club last month but never became a member because of his behaviour and a “bizarre” message on his voice mail.

When Lead Valley Range owner Glenn Rotkovich called to invite Holmes to a mandatory orientation, he said he heard a message on Holmes’ voice mail that was “guttural, freakish at best.”

He told his staff to watch out for Holmes at the orientation and not to accept him into the club, Rotkovich said.

Officials at the University of Colorado Anschutz Medical Campus were looking into whether Holmes, a former doctoral student in neuroscience, used his position in a graduate program to collect hazardous materials.

Holmes’ reasons for quitting the doctoral program in June remained a mystery. He recently took an intense, three-part oral exam that marks the end of the first year. University officials would not say if he passed, citing privacy concerns.

Ritchie Duong, a friend who has known Holmes for more than a decade, told the Los Angeles Times that he last saw Holmes in December and he seemed fine.

Academics came easily to Holmes, Duong said. “I had one college class with him, and he didn’t even have to take notes or anything.”

The family’s pastor recalled a shy boy who was driven to succeed academically.

“He wasn’t an extrovert at all. If there was any conversation, it would be because I initiated it, not because he did,” said Jerald Borgie, who last spoke with Holmes about six years ago.

Sunday was a day for healing and remembrance in Aurora. Several thousand people attended a prayer vigil, and President Barack Obama visited with families of the victims.

Obama said he told the families that “all of America and much of the world is thinking about them.”